Commonly owned, concurrently filed application Ser. No. 09/567,797 entitled xe2x80x9cPower Circuit Breaker with Air Gap Between Molded Insulative Casing and Grounded Barrier Insulating Operating Mechanismxe2x80x9d.
1. Field of the Invention
This invention relates to power circuit breakers and particularly to a compact power circuit breaker having a molded insulative casing with a grounded barrier between the pole mechanisms and the operating mechanism providing the dead front.
2. Background Information
Power circuit breakers for systems operating above about 1,000 volts typically utilize vacuum interrupters as the switching devices. The vacuum interrupters for each phase must be adequately electrically isolated from each other for the operating voltage. Where sufficient space is available, the vacuum interrupters and associated conductors can be spaced apart sufficiently to achieve the required isolation. For the higher voltages, or for a more compact arrangement, each vacuum interrupter is housed in a separate pod molded of an electrically insulative material such as a polyglass. These molded pods in turn are bolted to a metal box containing the operating mechanism. The metal box is grounded to isolate the operating mechanism from the line voltage of the power circuit. Manual controls for the operating mechanism are accessible at the front face of the metal box. The metal box provides structural support for the separately housed vacuum interrupters and the operating mechanism. As the metal box is grounded, it also provides a xe2x80x9cdead frontxe2x80x9d for the breaker which increases protection of the human operator from the high voltage of the power circuit. This is particularly important if there is a fault within the power section of the breaker. The metal box also provides electromagnetic shielding where the operating mechanism includes sensitive electronics such as an electronic trip unit.
There are some circuit breakers at the lower end of the power breaker voltage range which house the phase vacuum interrupters in bays within a single piece molded insulative casing. However, these circuit breakers also use a metal box to house the operating mechanism, and to provide a dead front and electromagnetic shielding.
A need has been identified for reducing the size of power circuit breakers in the above 1,000 volt range. However, this also requires reducing the size of the operating mechanism. A smaller operating mechanism delivers less energy which presents a challenge. A considerable amount of energy is required to close a power breaker into the sizable current in a distribution system in which loads are turned on. The ductile steel boxes in which the operating mechanisms are currently supported absorb a substantial amount of energy in deflection and distortion, but the mechanism is sufficiently robust to accommodate such losses. The smaller operating mechanisms required for the compact power circuit breaker cannot afford such losses.
There is a need therefore for an improved compact power circuit breaker. There is a more particular need for such a compact power circuit breaker which can function with a smaller, less powerful operating mechanism.
There is a related need for such a compact power circuit breaker which is structurally stiff to minimize mechanical losses.
There is a further need for such a compact power circuit breaker which incorporates a dead front to protect the operator from the line voltage and the operating mechanism from electromagnetic interference.
These needs and others are satisfied by the invention which is directed to a power circuit breaker having a molded insulative casing with a plurality of pole cavities each housing separable contacts in a pole mechanism. Preferably, the pole mechanism is a vacuum interrupter. The circuit breaker also includes an operating mechanism for opening and closing the separable contacts of the pole mechanisms. This operating mechanism is supported by the molded insulative casing. A grounding system includes a grounded barrier which is disposed between the pole mechanisms and the operating mechanism to provide a dead front for the circuit breaker and to protect the operating mechanism from electromagnetic radiation. The molded insulative casing provides the electrical isolation which allows the pole mechanisms to be mounted closer together and also provides a stiff structural mount for the operating mechanism.
In a preferred embodiment, the molded insulative casing has a rear section incorporating the pole cavities, and a forward section mounting the operating mechanism, with the grounded barrier sandwiched in between. The rear section and forward section of the molded insulative casing are clamped together, with the grounded barrier between, by fasteners extending through apertures in the grounded barrier.
The grounded barrier is an electrically conductive substantially planar member which can extend laterally beyond the molded casing, both horizontally and vertically to interface with the dead front of the switchgear assembly in which the circuit breaker is installed. Alternatively, the planar member has forward extending flanges along side edges.
The grounding system can also include an electrically conductive pan which extends under the molded insulative casing. This pan and the grounded barrier are electrically connected together and to ground. In the exemplary embodiment of the invention, the operating mechanism includes electrically conductive operating linkages mounted in and electrically grounded to the electrically conductive pan, and mechanically connected to the pole mechanisms to open and close the separable contacts.
The circuit breaker has a cover enclosing the operating mechanism with openings for access to manual controls. Preferably, this cover is a molded insulative cover.